1. Field of the Invention
The invention relates to an electromagnetic flowmeter particularly for in vivo measurement of blood flow quantities.
2. Description of the Prior Art
Electromagnetic flow measurement makes use of the electrical voltage which arises when a flowing fluid having charge carriers is brought into a magnetic field oriented perpendicularly to the flow direction. The resulting electrical field and, thus, the electrical voltage is then perpendicular to the plane which is defined by the flow direction and by the magnetic field and can be monitored with suitable electrodes and supplied to evaluation circuits. The generated voltage is thereby proportional to the flow rate, i.e. to the volume of fluid which flows through the cross-section of the fluid conductor per second.
This measuring principal proves superior for physiological examinations of blood flows in practice: the measuring method does not influence the flow, rapidly changing flows can be identified, the measuring probes are relatively small and can be implanted at blood vessels for longer measurements, they require no damage to the blood vessel, the measured signal is largely independent of the flow profile and of the physical properties of the blood, and fast changes of the blood flow can aiso be acquired. Examples of such probes are disclosed, for example, in the book "Transducers For Biomedical Measurement" by R. S. C. Cobbold which was published by J. Wiley in 1974. These probes differ from one another on the basis of the manner in which the magnetic field to be applied perpendicularly to the axis of the blood vessel is generated. Such a probe or such a flow-through measuring means is also known from German OS No. 1923071. All of these known probe forms have in common the use of magnetic coils with or without a low-retentivity (ferromagnetic) core member are employed, these generating an alternating magnetic field having a frequency of about 400 Hz and above (measuring errors as a consequence of electro-chemical polarization processes at the electrodes are thereby largely avoided). For smaller blood vessels, C-shaped or U-shaped magnetic coils are employed or coils which contain a toroidal, low-retentivity body whose two legs are charged with magnetic coils having opposite winding direction, so that a magnetic field arises in the inside of the toroidal member.
These known probes for electromagnetic blood flow measurement, however, still exhibit a number of disadvantages: they do not supply an adequately high measured signal given extremely small blood vessels, the excitation methods presently employed for the alternating magnetic field lead to disturbances of the measured signal, the datum line balancing is difficult, the dimensions of the probe are still too large for many implantation purposes, and the mechanical and electrical stability of the diverting cable from the probe to the outside of the body still leaves something to be desired.
For physical reasons, these disadvantages cannot be eliminated by improving the known probe forms. A miniaturization of the probe given the same excitation current for generating the magnetic field leads to an increase of the stray power since this is proportional to the reciprocal of the coil cross section. Given implanted probes, however, high local heating cannot be accepted. Given measurements at small blood vessels having a correspondingly small flow quantity, on the other hand, a boost of the magnetic field is required since the measured signal is proportional to the flow stream. A higher coil current, however, leads to a greater, unacceptable increase of the stray power which rises proportional to the square or the coil current.
Disturbances of the measured signal due to interaction with the coil current which also flows during the measurement probes are likewise difficult to govern given small probes having closely adjacent conductors. For electromagnetic flowmeters which are employed outside of the biological-medical field, German OS No. 30 29 791 has proposed that a permanent magnet which is periodically remagnetized be employed instead of a magnetic yoke with low-retentivity material. The measurement of the electrical flux signal is carried out in those time segments in which no remagnetization current is flowing. The measurement head probes therein, however, have extremely large dimensions and cannot be used for implantation purposes; moreover, it cannot be operated with the frequencies of about 400 Hz required for blood flow measurements since high eddy currents arise in the permanent magnet material at these frequencies. The generation of thermal energy connected therewith would heat the measuring probe to a temperature which would be unacceptable for a physiological blood flowmeter. The permanent magnet materials proposed therein, moreover, do not have a rectangular hysteresis loop as required for a measuring probe of the species proposed here. The inductivity of the coil with a magnetic core likewise hardly allows the required edge steepness of, for example, 5.multidot.10.sup.7 A/sec due to its large current pulses.
Due to the solid execution of the permanent-magnetic pole members, a physical effect which has an extremely disadvantageous influence arises. Different hysteresis loops are traversed in the volume material of the permanent-magnetic, solid pole members in different depths because the cructial magnetic field penetrates relatively slowly into the volume material, this being caused by the unavoidable self-induction. The currents induced by the self-induction also attenuate the penetrating field. It follows therefrom that, due to the relatively long relaxation time of the magnet, a magnetic field which is truly chronologically constant is never obtained. Noise effects which deteriorate the ratio of noise signal to useful signal, i.e. the signal-to-noise ratio, are superimposed on the measurement.
The relatively long relaxation time of the solid permanent magnet during the switch-over phase triggers cross-talk voltages in the measuring line as well as induced voltages on the line for the supply voltage, making the zero point of the measurement unstable from synchronous voltages.